Production of extruded polytetrafluoroethylene articles



Patented Oct. 25, 1949 PRODUCTION OF EXTRUDED POLYTETRA- FLUOROETHYLENEARTICLES Stephen Bernard Bogese, Upper Montclair, N. J.,

assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., acorporation of Delaware No Drawing. Application February 20, 1948,Serial No. 9,943

4 Claims.

The invention relates to a method of preparing improvedpolytetrafluoroethylene' articles and more particularly to a method ofpreparing various polytetrafluoroethylene articles having superiorproperties by means of an extrusion method.

Methods of preparing polytetrafiuoroethylene are disclosed in U. S.Patent 2,230,654, U. S. Patent 2,393,967, and U. S. Patent 2,394,243.Polytetrafluoroethylene is a polymeric material highly resistant tocorrosion and oxidation, and is exceedingly valuable for the manufactureinto commercially useful articles.

Heretofore, useful articles have been fabricated fromtetrafiuoroethylene polymer by batch-wise processes and by a continuousextrusion process for the fabrication of rods, tubes, and coatedarticles, as described in pending application S. N. 691,406, filed onAugust 17, 1946, by A. J. Cheney, now U. S. Patent No. 2,456,621,wherein finely divided polytetrafiuoroethylene is prebaked at atemperature of 300 C. to 500 C. before extrusion through a die in whichthe polymer is sintered at a temperature above 327 C. While passingthrough the said die.

An object of this invention is to provide polytetrafiuoroethylenearticles having superior properties. A further object is to provide apolytetrafiuoroethylene coating on objects such as wires and the likehaving unusually long life under high dielectric stress as well as beinguniform, continuous, flexible and tough. Other objects will be apparentfrom the detailed description of the invention given hereinafter.

The above objects are accomplished according to this invention byintimately mixing finely divided unbaked polytetrafluoroethylene withfinely divided polytetrafluoroethylene prebaked at a temperature of 300C. to 500 C. in such proportions that the resulting mixture will containfrom 25 percent to 95 percent prebaked polytetrafluoroethylene powder;passing said mixture through a die having an internal contour ofsubstantially the size and shape of the desired finished article; andbaking the polymer while passing through the die at a temperature above327 C. until sintcred. In extruding a coating of this polymer on objectssuch as wires and the like, the object to be coated is fed into anextruder and carried along the path of the extruding polymer by thetensional pull of the polymer thereon. The said finely divided unbakedpolytetrafluoroethylene, hereinafter known as unbakedpolytetrafluoroethylene powder, is the product of the process describedin application S. N. 714,811, filed December 7, 1946, by W. D. McKinley,or is the dried suspensoid of the process described in application S. N.713,385, filedNovember 30, 1946, by M. M. Renfrew.

It is advantageous, although not essential, to heat the polymer mixturebefore passing the same through the die. This pre-heating step, whichmaterially shortens the time required for the entire operation bydecreasing the time required in the subsequent baking step, may beaccomplished in the extruding mechanism by conventional heating means ata temperature below 327 C. and preferably at a temperature ofapproximately 200 C. to 300 C.

It has been found that by mixing unbaked finely dividedpolytetrafluoroethylene powder and prebaked finely dividedpolytetrafluoroethylene in such proportions that the resulting mixturewill contain from 25 percent to percent finely divided prebakedpolytetrafiuoroethylene, the said mixture can be as readily fed into anextruding machine as the prebaked polymer itself, whereas it isimpractical to feed unbaked polymer to such an extruding machine. Whileany blend in the above range may be used, it is preferred that theresulting mixture contain prebaked polymer in an amount of from 40percent to 90 percent by weight of the mixture. The properties of theextruded articles obtained from the above polymer mixture are greatlysuperior to those articles extruded from the prebaked polymercomposition, and this improvement is most readily observed bymeasurement of the electrical properties of coated electricalconductors. The order and magnitude of the improvement in electricalproperties is illustrated by the following examples:

Example I Polytetrafluoroethylene powder, prebaked for 60 minutes at 3800., was mixed with unbaked polytetrafluoroethylene powder so that theresulting mixture contained 85 percent by weight of prebaked finelydivided polytetrafluoroethylene. The blend was fed into a screw stuffermaintained at a temperature of 315 C. to which a cross-head wasattached. The screw stuffer extruded the blend into the cross-head andthen into a 0.298-inch diameter mild steel tubular die. Into this diethrough said cross-head was introduced 7 strands of silverplated copperwire which had been twisted together to form a 0.086-inch diameterstranded wire conductor. The crosshead and the 26-inch long baking zoneof the die were maintained at a temperature of 400 C. The extrusion rateof the 0.275-inch diameter coated conductor was at the rate of 8 feetper hour.

A sample of this coated conductor was subjected to an electrical stressof 18,000 volts for more than 180 minutes before failure of the coatingoccurred. A conductor, coated with polytetrafluoroethylene which hadbeen prebaked for 60 minutes at 380 C. and not mixed with any unbakedpolymer was subjected to the same elec- 3 tricalstreasandfailm-eoithecoaiingresoited in 0.1 minute.

Example I! A center hole 0.093-inch in diameter was drilled alongthelongitudinalaxisofthescrewstuiler of Example I. A mild steel tubular die0.208-inch indiameterwas attachedtothescrewstnflerinsuchamannerthattbediewillhaveacommon axis with the screw.Polytetraiiuoroethylene powder prebaked at 800 C. for 8 hours wasblended with unbaked polytetraiiuoroethylene powder so that theresulting mixture contains 75 percent by weight of the prebakedpolytetrafiuoroethylene. The blend was then fed into the screw stuiler.Stranded wire of the type used in Example I was fed into the center holeof the screw until the wire reached the point at which the polymer waspassing from the screw stufluinto the die, then the wire was carriedthrough the die by the polymer. The 18-inch long baking zone of the diecan be maintained at a temperature of 480 C. The 0.275-inch diametercoated conductor may be extruded at the rate of 12 feet per hour.

A sample of this coated conductor may be subjected to an electricalstress of 18,000 volts for 152 minutes before failure results, while thetime for failure of a coated conductor prepared under identicalconditions from polytetrailuoroethylene powder prebaked for 8 hours at300 C. and not mixed with non-baked finely dividedpolytetrafluoroethylene was 0.2 minute.

The following examples are specific illustrations as to the continuousextrusion of rods and tubes.

Example III Polytetrafluoroethylene powder, prebaked for minutes at 380C. was blended with unbaked polytetrafluoroethylene powder in such aproportion so that the resulting mixture constitutes 90 percent byweight of the mixture of finely divided prebaked polytetrafluoroethylenepowder. The polymer blend was fed into a screw stufier to which isattached a l-inch diameter steel tubular die having a baking zone24-inches long which was maintained at a temperature of 380 C. A rod%-inch in diameter was extruded at the rate of 3 feet per hour.

Example IV Polytetrafluoroethylene powder, prebaked at 360 C. for onehour, was blended with unbaked finely divided polytetrafluoroethylene insuch a proportion that the resulting mixture contained percent by weightof the mixture of finely divided prebaked polytetrafluoroethylenepowder. The blend was fed into a screw stuffer which extruded the powderinto a die designed for the production of tubes. This die comprises asteel tube 1%-inches inside diameter having a %-inch diameter metal coresuspended therein. The 18-inch long baking zone of the die wasmaintained at a temperature of 380 C. A polytetrafluoroethylene tube1%-inches outside diameter by %-inch inside diameter was extruded at therate of 2 feet per hour. I

It is to be understood that the above examples are merely illustrativeof the embodiments of this invention and that the invention broadlycomprises blending fineiy divided unbaked polytetrafluoroethylene powderwith finely divided polytetrafluoroethylene prebaked at a temperature of300 C. to 500 C. in such proportions that the resulting mixture willcontain from 25 percent to 05 percent finely divided prebakedpolytetrsfluoroethylene powder, passing this blendedpolymermixtureandtheobiecttobecoatedif a coating operation is involved,simultaneously through a die having an internal contour of substantiallythe size and shape of the desired finished article, and baking thepolymer while passing through the die at a temperature above 327' C. butnot in excess of 500 C. It is preferred that the mixture contain the,flnely divided prebaked polymerin an amount of from 40 percent topercent by weight of the mixture.

The extrusion of unbaked polymer is most desirable, however. it has notbeen found possible to feed unbaked finely dividedpolytetraiiuoroethylene into an extruding mechanism because of theinherent resistance to flow of the finely divided particles of theunbaked polymer which prevent the feeding of the polymer into the screwstuilfer. By the addition to the unbaked polytetrafluoroethylene powderof at least 25 percent by weight of the resulting mixture of prebakedpolytetratluoroethylene powder the mixture feeds satisfactorily into thescrew stufler.

The dies referred to herein usually serve the simultaneous dual purposeof dies and baking zones, the polymer being baked while passing throughthe heated die. However, the baking zone may include more than onesection having the same or different inside diameters and maintained atthe same or different temperatures. In the event that two or moresections of different diameters are employed, the smallest diametersection is normally placed nearest the extruder and serves as a die andbaking zone, while the larger diameter sections serve primarily asbaking zones.

The baking step is essentially a sintering operation, while slnteringmay or may not be accomplished in the prebaking step. To sinter thepolytetrafluoroethylene, the temperature must be raised above 327 C.There is no particular advantage however, in exceeding a temperature of500 C. more often it is disadvantageous, and usually the slntering canbe effected more satisfactorily at a considerably lower temperature. Theheating operations involved in this invention may be accomplished bymeans of electric heaters, gas heaters, a liquid, e. g. hot oil, amolten metal or alloy, or a mixture of molten inorganic salts. Electricheaters are preferred especially for carrying out the baking step.

An essential feature of this invention is the blending of prebaked andunbaked polytetrafluoroethylene. While it has not been found possible topractically extrude unbaked polyietrafiuoroethylene into articles ofuniform density characterized by freedom from flaws, prebaked polymer isextruded into articles of uniform density. However, these articlesformed from prebaked polytetrafluoroethylene per se have physicalproperties inferior to the articles obtained by the extrusion of amixture containing unbaked and prebaked polymer powder in suchproportions that the resulting mixture will contain from 25 percent topercent prebaked polytetraiiuoroethylene powder.

It has been found that although the powdered mixture of prebaked andunbaked polytetrafiuoroethylene polymer may be extruded through the dieby means of a ram moving in a reciprocating manner and forcing thepowder directly into the die, a preferred method of accomplishing thisstep is by means of a screw stuiier. As the polytetraiiuoroethylenemixture is being extruded through the die, there is a tendency to formloosely compacted articles because of insuflicient compacting pressure.This tendency is overcome by using a longer die or by applying amechanical break to the article being extruded after it leaves the die.Although this compacting pressure is not critical over a very wide rangeof pressure, e. g. about 50 to 10,000 pounds per sq. inch, this pressureand particularly the minimum pressure does have some bearing on thequality of articles produced. The upper pressure which may be employedis limited perhaps more by the strength of the apparatus used than anyother factor. The pressure to be employed will depend somewhat on thetype of article being produced. A suitable pressure under thecircumstances employed may be readily determined by those skilled in theart.

The process of the present invention is applicable both to thepolytetrafluoroethylene mixture per se and to mixtures of the saidpolytetrafluoroethylene mixture with other components such as finelydivided copper, tin, iron, lead, brass, bronze, graphite, asbestos,silica, calcium chloride, calcium fluoride, sodium fluoride, ammoniumchloride, ammonium nitrate, titanium dioxide or the like. These additivematerials are usually added in the powder form, but in some instances,can be added in the form of fibers, for example, asbestos, glass, andthelike.

The process is also applicable to copolymers of tetrafiuoroethylene withother polymerizable compounds such as isobutylene or ethylene andparticularly copolymers containing substantial amounts oftetrafluoroethylene although more convenient means may be available forfabricating articles from copolymers, especially those containing arelatively small portion of tetrafluoroethylene.

A further application of the tubes and rods produced according to theinstant invention is for the making of sheets by placing the articles ina lathe or sheeting machine and turning ofi a continuous sheet ofmaterial. The articles are well suited for this purpose because they areuniform in size and shape and because there is no waste due to flaws inthe said articles. Of course, the articles may be made in various sizesfor subjection to subsequent sheeting and the like operations.

As will be apparent, semi-finished articles may be produced according tothis invention and thereafter machined or otherwise treated to give thedesired finished articles. Further, the finished articles of the instantinvention may be made into more complex shapes or otherwise modified bymachining or treating in other ways to give the desired finishedarticles.

Another advantage of this invention is that scrappolytetrafluoroethylene, resulting from articles produced by this orother methods in which temperatures above 327 C. but not in excess of500 C. were employed, may be finely divided and used instead of or aspart of the prebaked polymer for making articles according to thepresent invention, thus providing a process for utilizing scrappolytetrafluoroethylene which was heretofore discarded.

As many apparently widely different embodi ments of this invention maybe made without departing from the spirit and scope thereof, it is to beunderstood that the invention is not limited to the specific embodimentsthereof except as defined in the appended claims.

Number I claim:

1. A process for making polytetrafluoroethylene articles which comprisesmixing finely divided unbaked polytetrafluoroethylene with finelydivided polytetrafluoroethylene prebaked at a temperature of 300 C. to500 C. in such proportions that the resulting mixture will contain from25 percent to 95 percent by Weight of said prebakedpolytetrafluoroethylene; passing the said mixture through a die undercompacting pressure, the said die having an internal contour ofsubstantially the size and shape of the desired finished article; andbaking the said polytetrafluoroethylene at a temperature between 327 C.and 500 C. until sintered.

2. A process for making polytetrafluoroethylene articles which comprisesmixing finely divided unbaked polytetrafluoroethylene with finelydivided polytetrafluoroethylene prebaked at a temperature of 300 C. to500 C. in such proportions that the resulting mixture will contain from25 percent to 95 percent by weight of said prebakedpolytetrafiuoroethylene, passing the said mixture through a die undercompacting pressure, the said die having an internal contour ofsubstantially the size and shape of the desired finished article; andbaking the said polytetrafiuoroethylene while passing through the die,at a temperature between 327 C. and 500 C. until sintered.

3. A process for coating objects with polytetrafluoroethylene whichcomprises mixing finely divided ,unbaked polytetrafluoroethylene withfinely divided polytetrafluoroethylene prebaked at a temperature of 300C. to 500 C. in such proportions that the resulting mixture will containfrom 25 percent to 95 percent by weight of said prebakedpolytetrafluoroethylene; simultaneously passing the saidpolytetrafluoroethylene mixture and the object to be coated through adie under compacting pressure, the said die having an internal contourof substantially the size and shape of the desired finished object; andbaking the said polytetrafluoroethylene while passing through the die,at a temperature between 327 C. and 500 C. until sintered.

4. A process for coating wire with polytetrafluoroethylene whichcomprises mixing finely divided unbaked polytetrafluoroethylene withfinely divided polytetrafluoroethylene prebaked at a temperature of 380C. in such proportions that, the resulting mixture contains percent byweight of prebaked polytetrafluoroethylene powder; heating the saidmixture to a temperature of 315 0., simultaneously passing the saidheated polytetrafluoroethylene mixture and the said wire through a dieunder compacting pressure, the said die having an internal contour ofsubstantially the size and shape of the finished article; and baking thesaid polytetrafluoroethylene while passing through the die, at atemperature between 327 C. and 500 C. until sintered.

STEPHEN BERNARD BOGESE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PA'I'ENIS Name Date Alfthan May 14, 1946 OTHER REFERENCESYelton, Telfion, Plastics and Resins, May

76 1946, pages 14-16 and 36.

